Dry blanket for metal baths



DRY BET FOR METAL BATES William Spowers, In, Maplewooall, N. J.

No Drawing. Application March 6, 1935, serial N; 9,611

This invention relates to covers for zinc galvanizing tin, lead or other such baths.

When zinc is employed for 6 Claims.

improvements in (spelter) baths, or for galvanizing iron or other metals it is commonly maintained molten at temperatures of about 420" while through the bath. tendency of the zinc is the material to these conditions, with the result that much zinc is lost. In order to avoid been proposed to cover the bath, but tive cover are so many,

this disadvantage, it has employ certain materials to the requirements of an effecvaried and diflicult to satisfy that prior proposals have not been entirely satisfactory in one or more important respects.

Among the desired properties of a cover for this purpose is the prime requisite that the cover be effective for substantially of air to the zinc. should not decompose At the same or volatilize so rapidly as preventing access time the cover to require frequent replacement. Moreover, the

cover should be free flowing, should not stick to the galvanized metal as it is withdrawn through the cover, should not lump or cake, should not dull the finish of the galvanized material, should not congeal the zinc, and should not give ofi fumes which are noxious or highly disagreeable.

The cover should not only meet these requirements but also be inexpensive, easy to apply to the bath, simple to handle, and capable of being added to previous quantities of covering material already on the bath and of mixing therewith.

A feature of the present the provision of a novel and invention resides in improved coating which satisfies these varied and conflicting requirements in a superior manner.

A further feature lies in providing a cover of ,finely divided material, and more particularly in providing a cover of carbon treated to improve its capacity for remaining on the bath a long period of time without burning away,

and with the ability to prevent oxidation of the zinc and at the same time permit the galvanizing operations to be conducted without the cover deleteriously or adversely afiecting galvanized articles.

Still another feature is the a cover. which does not stick the properties of the provision of such to the galvanized metal to such extent as to require anything more than a simple joining, brushi operation to remove any from the galvanized articles,

ng or air blowing particles of the cover or in other words so that no costly extra operations are required C. to 475 C., be galvanized is drawn It is well known that the to oxidize rapidly under on account of the use of the cover, such as scrubbing or scraping of the articles.

A more specific feature resides in the provision and employment of a cover composed primarily of carbon particles prepared from raw carbonaceous material by treating to remove most if not all of those constituents which tend to burn at temperatures lower than the melting point of the metal bath, or in other words the provision of a carbon that is more stable and otherwise effective than ordinary charcoal or cok A more detailed feature resides in the discovery that carbon which has been activated is markedly superior as a zinc bath cover, and that the superiority of such materials may be still further enhanced by incorporating in them certain inorganic substances.

Other features reside in the provision of a novel, covered spelter bath and process of employing the same.

Still other features, objects and advantages of the present invention will in part become apparent and in part be pointed out in connection with the following detailed description of examples illustrating the nature of the present development.

As mentioned above, galvanizing baths involve the use of a molten mass of zinc upon the upper surface of which the present cover may be employed in a layer of suitable thickness, for instance in the order of one-half inch to two inches.

The cover may be employed over the entire bath, or part of the bath may be covered in other ways and the present cover may be employed only at the outlet end of the bath where the galvanized articles are drawn out between wipes or in other usual manners.

For this purpose the cover may advantageously and conveniently be prepared by taking ordinary charcoal or other carbonaceous material (such as peat, sawdust, wood pulp, and the like) and heating it to a high temperature, for instance about 900 C. to 1000 C., with limited access of air, or in an atmosphere of steam or carbon dioxide, for a period suflicient to drive off most if not all of the low burning or volatile constituents of the carbonaceous material. Processes of the type commonly used for making activated carbon are ordinarily satisfactory. The degree and extent of treatment may vary considerably, however, for it may not be necessary to treat the carbon to the extent required for completely activating it in the extent to which the treatment is carried may be relatively prolonged when the metal bath temperature is high, and may be comparatively short where the temperature of the metal bath is low.

While various carbonaceous materials may be employed as the raw material from which the improved carbon is prepared, materials such as lignite appear to be superior for present purposes. Moreover, while the treatment of the carbonaceous material may also vary, the raw material and process employed are preferably such that the product is relatively soft and has a low density as compared to certain hard, high density carbons now on the market.

Merely by way of specific example, excellent results may be secured from activated carbon prepared according to the process of United States Letters Patent to Russell W. Mumford, No. 1,286,187 issued November26, 1918. In accordance with this patent, material of carbonaceous character may be treated by heating it in a retort, gradually at first, until the temperature is carried up to around 900 degrees C. The raw carbonaceous material may be preliminarily comminuted to a suitable fineness and may then be mixed with a colloidal or an inorganic material adapted to liberate a gas when heated, or with both such types of material. The patent indicates advantages in the use of each of these types of materials. However, the present invention is not to be limited to the use of either of these types of material, or for that matter, to any other details of the Mumford patent referred to, the reference to this patent merely being for the purpose of indicating one process which may be employed to produce an excellent type of activated carbon by expelling from raw carbonaceous material cer-= tain undesirable ingredients. Moreover, while the word activated is employed in describing the preferred form of carbon according to the present invention, it may be that the porosity incident to activated carbon is not entirely essential, pro= vided the product does not burn readily at the temperatures to which it is subjected upon application to molten metal baths at temperatures of, say, 420 C. to 475 C., or even possibly somewhat higher.

Incidentally, it may be noted that there are many processes for activating carbon and the present invention in its broader aspects is not limited to the use of any one of these processes. However, by way of example, reference is made to United States Patent 1,923,918 issued August 22, 1933, to Davis and Sanders, which illustrates another manner of activating carbon. Process of the character indicated appears to produce an activated carbon which is especially suited to present requirements, being capable of withstanding the temperatures of a zinc bath for as much as eighteen hours or even more, whereas ordinary charcoal burns or ashes sometimes immediately and generally within one-half to two hours.

'The carbon which I prefer to employ is preferably not too finely divided, so that excessive tendency to burn or ash is avoided, and at the same time is sufficiently fine so that a good blanket or cover of uniform consistency is provided, capable of preventing substantial access of air to the zinc. For example, the carbon is ad vantageously of such size as to paw a four mesh screen but not a twelve mesh screen, although it may be more finely divided, e. g., so that it all passes a twenty mesh screen but most of it is retained on a forty mesh. While the coarser grades within this range are excellent for this aocaaea purpose, it will be appreciated that the range is preferential and not absolutely a criterion for all purposes and conditions. However, the finer grades have a greater tendency to burn than the coarser grades.

In order to further reduce the tendency of the carbon to burn, it may be treated either before or after, but preferably after, the activating process, with a solution of an inorganic substance such as boric acid, which is adapted to impregnate the carbon with inorganic material which tends to prevent the carbon from burning. Such substances as boric acid, and to a less extent oxalic acid, ammonium chloride and zinc ammonium chloride, tend to prevent burning of carbon when incorporated in the carbon by washing the same with solutions of these compounds. When boric acid is employed, high grade activated carbon prepared from lignite will absorb as much as three times its weight of this inorganic material which tends to prevent the carbon from burning. However, much less may be used to advantage, for instance 50% by weight of inorganic material. Boric acid is superior to the other inorganic materials mentioned and tends to improve even inferior grades of carbon or charcoal so that they do not burn over substantial periods when placed on metal baths. On the other hand, the superior grades of activated carbon which have been treated to expel the low burning ingredients may not be very much improved by impregnation with salts or acids. Accordingly, this step is not required in many cases.

While there is no wish to be bound by any particular theory of operation, it is suggested that when activated carbon particles are employed as a cover for molten metal baths, some particles nearest the bath may be initially converted to carbon monoxide. Sometimes this gas is absorbed by the particles of carbon through which it seeks to rise. If not absorbed then there is formed a film of inert gas between the outside air and the molten metal, with the result that the air is prevented from reaching either the metal bath or the carbon particles which are next to the bath and hence most highly heated. Practically, some of the carbon monoxide is absorbed and other portions thereof form the layer of gas and operate as above described. Whatever the theory, however, the result is that the metal is prevented from oxidizing and at'the same time the carbon is prevented over long periods from rapidly ashing or burning. For instance, it has been found that when a layer of activated carbon is spread on a zinc bath and maintained at a temperature of about 460 degrees 0., the activated carbon showed no tendency to burn. Moreover, the carbon did not glow and only a small portion of it was converted to ash. When activated carbon impregnated with boric acid was similarly employed, the carbon showed only traces of ash.

As mentioned above, it may be that carbon free from low burning constituents, whether it is porous or not, will give satisfactory results for present purposes. Accordingly, while the process of expelling such constituents by activation produces high porosity, and while the products of such a process have proved excellent, it is contemplated that satisfactory relatively non-porous carbon may be producible by methods which are not strictly activationprocesses, and such high burning carbons are therefore not excluded from the broader aspect of the invention.

'lihe terms and expressions which have bee employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents oi the features shown and described, or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is: 1. it cover for molten metal baths, comprising a layer of activated carbon impregnated with boric acid.

2. A cover for molten metal baths, comprising a layer of activated carbon impregnated with boric acid, the horic acid constituting about onehalf to three times the Weight of the carbon.

3.1a cover for molten metal baths comprising activated carbon of relatively low density and high porosity, said carbon comprising particles which pass a a mesh screen but not a 12 mesh 0 screen.

7 cover having therein 4'. A cover for molten metal baths comprising activated carbon of relatively low density and high porosity, said carbon comprising particles which pass a 4 mesh screen but not a 12 mesh screen, said particles being impregnated with inorganic material.

5. A cover for galvanizing baths comprising a layer of particles of carbon of a size such that the major portion of them will pass a i mesh screen, but not a 40 mesh screen, said particles having been treated at a temperature of about 900 C. to 1000 C. in an atmosphere of steam, and containing inorganic material in more than a trace form.

6. In a process of protecting galvanizing baths against oxidation, the step of placing thereon a activated carbon impregnated with inorganic material.

VVHLIAM H. SPOWERS, JR. 

